Engine analyzers provide mechanics with a tool for accurately checking the performance of the ignition system as a measure of the overall engine performance. Signal detectors (“test probes”) are widely used in diagnosing defects and anomalies in internal combustion engines. A test probe is, for example, placed adjacent to a test point such as a ignition coil or ignition wire, and the test probe communicates the signal back to a motor vehicle diagnostic apparatus. Information obtained from the test probe, such as spark plug firing voltage and duration, can help a mechanic determine if a spark plug associated with the ignition coil is functioning properly.
FIG. 1a illustrates a capacitive signal detection system. Ignition coil 110 is, essentially, a transformer having a very large turn ratio, typically between 1:50 to 1:100, between the primary and secondary, which transforms the low voltage in a primary winding provided by the sudden opening of the primary current to a high voltage in a secondary winding. Ignition coil 110 is connected to the center or coil terminal (not numbered) of distributor cap 114 by an insulated wire 112. High voltage from the ignition coil 110 is distributed from the coil terminal to side or spark plug terminals of the distributor cap 114 by means of a rotor which distributes the spark to each spark plug terminal at a predetermined timing in a manner known to those skilled in the art and provided in standard technical manuals. The spark voltage provided to the spark plug terminals is, in turn, provided to a corresponding spark plug 122 via insulated wires 118.
At each cylinder, the resulting electric discharge between the spark plug electrodes produces a spark which ignites a fuel-air mixture drawn or forced into the cylinder and compressed to an explosive state, thereby driving a piston in the cylinder to provide power to an attached crankshaft. Analysis of ignition waveforms to evaluate engine performance can be performed by capacitively coupling a capacitive signal pickup 124 to the spark plug wire 118. The capacitive signal pickup 124 is conventionally wrapped around or clipped to wire 118, at one end, and is connected to measuring device 128, at another end, through a wire or coaxial cable 126. The total capacity measured by the pickup 124 is used, in combination with a conventional capacity divider circuit, to determine the wire 118 voltage in a manner known to those skilled in the art.
More recently, ignition systems have evolved to one coil per cylinder or one coil per cylinder pair (a direct ignition system (DIS) or hybrid), and may not have any spark plug wire at all. Such spark ignition systems incorporate an ignition coil over each plug or an ignition coil near each plug as shown, for example, in FIG. 1b. High voltage generated at the secondary coil 164 by means of the primary coil 162 and magnetic iron core 160 is routed through the output of the secondary coil through various conductive elements to a conductive output, such as a spring 169, and to the spark plug (not shown) housed within spark plug cap 160. Igniter 168 is a switch that opens after current has been flowing in the coil. This transient develops a large voltage on the primary which is increased by transformation through secondary coil.
FIG. 1c shows a coil-over-plug (COP) assembly having ignition coil 140, spark plug 150, and spark plug gap 151. This arrangement prevents application of the conventional technique implemented in FIG. 1a, since the high secondary voltage conductor is not as easily accessed as the wire 118 of FIG. 1a. For this configuration of COP, a coil-on plug signal detector assembly or sensor 141, such as taught by U.S. Pat. No. 6,396,277, issued on May 28, 2002, and assigned to the present assignee, which is incorporated herein by reference, may be used. The COP sensor 141 includes upper and lower conductive layers (not shown) affixed to and separated by substrate 144. The upper and lower conductive layers act, in one aspect, as a signal detector and as a ground plane. The upper layer is conductively coupled to an external signal analyzer device via wire 152 and the ground plane reflects a portion of the electromagnetic energy generated by the coil, thus serving to attenuate the strength of the signal observed at the signal detector layer to a level easily handled by conventional analyzers. The sensor 141 is clipped to the housing of the ignition coil 140 by a clip 147 attached to sensor housing 148.
In this arrangement, sensor 141 lies within a field of electromagnetic radiation emitted by coil 140 when the coil is transforming primary voltage into high-voltage for use by a spark plug. In operation, low voltage and high current are applied to the primary winding of ignition coil 140 for a predetermined time, and the primary winding generates an electromagnetic field that principally consists of a magnetic field (H). The secondary winding generates an electromagnetic field that is primarily an electric field (E) because it carries high voltage and low current. The lower conductive layer, which is placed adjacent a housing of the coil 140, is brought substantially to ground potential by virtue of such contact. A voltage potential, which could be positive or negative (generally negative for a COP system), is induced or otherwise developed across upper and lower layers 148, and may be measured at or received from the surface of the upper layer or signal detector layer. The voltage observed at the signal detection layer is proportional to the voltage at the terminal end of the secondary coil of coil 140. A signal taken from the signal detection layer may therefore be used in diagnosing ignition spark voltage characteristics, such as spark voltage or burn time, or other problems such as open wires or plugs or fouled or shorted plugs, in a manner known to those skilled in the art.
Despite the advancements realized by present coil-on plug signal detection devices, the sheer variety of ignition coil configurations make it difficult for any one sensor to find universal applicability. For example, the aforementioned sensor 141 may be less than optimal when the coil housing is shielded or otherwise configured to output a distorted or significantly attenuated signal. One example of this occurs in coil-on plug/coil-over plug assemblies bearing an igniter in a ferrous shielded box, which acts a shield for both electric and magnetic fields emanating from the core. Shielding is broadly considered to include any medium or combinations of mediums that serve to notably attenuate a field output from the coil-on plug assembly, even if such shielding was not itself a design consideration. Therefore, there is a need for a coil-on plug/coil-over plug signal detection device suitable for use in low-output ignition coil configurations.